PHOTOMETRY AND ELECTRIC LIGHTING. 321 



When the photometer has been thus properly adjusted its reading must be multi- 

 plied by the factor zj^irkr 2 to give the true spherical-candle power of a lamp, L, 

 which is being measured, z being the area of the zone of the reference sphere which is 

 associated with the horizontal beam, and 471-7-2 being the total area of the reference 

 sphere. 



The theory of the Matthews integrating photometer is here given in full on account 

 of the fact that this photometer is destined to come into general use. 



133. The problem of illumination. A room may be said to be 

 well lighted when the eye is easily able to distinguish, in minute 

 detail of perception, the various objects in the room. Complete- 

 ness of visual perception depends upon three conditions, namely, 

 (a) a sufficient brightness of the illumination ; (&) a proper loca- 

 tion of the light sources, so as to bring out that combination of 

 soft shadows which is so essential to the perception of form, and 

 (c) a proper composition * of the light, so as to bring out those 

 physical differences in objects which the eye perceives as varia- 

 tions of color. 



A room may be intensely illuminated by a single arc lamp, but 

 such illumination is ineffective, even when the eye is shaded from 

 the direct light of the lamp, because the excessive harshness of 

 the shadows renders the perception of form almost impossible ; 

 on the other hand a sufficiently intense and properly distributed 

 light, which contains certain wave-lengths in great excess, may be 

 ineffective because of the unusual or weak color effects produced 

 thereby. For example, the light of an ordinary kerosene lamp 

 is very deficient in the shorter wave-lengths ; these shorter wave- 

 lengths have much to do with the bringing out of blue and violet 

 tints ; and, consequently, a deep blue or violet piece of cloth 

 appears almost black by kerosene lamp light. False color val- 

 ues are produced in a very striking way by the light from the 

 mercury -vapor lamp on account of the almost complete absence 

 of the longer wave-lengths (red) in the light from this lamp. 

 The brilliant white light of the carbon arc lamp, on the other 

 hand, contains all wave-lengths in about the same proportion as 



* The composition of light refers to the relative intensities of the various wave- 

 length components of the light. 



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